Reservoir properties are sampled at well logs (wireline, LWD or cased-hole logs). Proper characterization of a reservoir, particularly for estimates of net rock volume, porosity volume, and original oil in place, requires an estimate of the vertical trend curves of shale volume, porosity, permeability, saturation, etc. and the uncertainty of these vertical trend curves. Vertical trend curve uncertainty is a key component of reservoir characterization that affects volumetric uncertainty and reservoir recovery forecasts. Vertical property trends, in particular, can significantly affect the amount of oil in place estimated to be above an oil-water contact, because for example, a fining-upward trend will place less reservoir rock in the upper portions of the model above the contact and, likewise, a coarsening-upward trend will place more reservoir rock above the contact. Vertical property trends, particularly trends in permeability, can also significantly affect the movement of oil, gas, steam, and water in a reservoir.
Typically a reservoir modeler will have no way to derive accurate vertical trend curve uncertainty for his model. Conventional statistical techniques of bootstrap are often used to assess the uncertainty of population statistics or property distribution (for example, as implemented in application Crystal Ball from Oracle Corp). However, conventional bootstrap methods assume incorrectly that each property data collected is an independent measurement. Spatial bootstrap methods do not assume data independence but these methods are conventionally used solely to determine the uncertainty of the mean of some property distribution. These methods are not used to determine the vertical trend curve uncertainty.
Therefore, there is a need for a method of determining uncertainty of a vertical trend curve such as, but not limited to, vertical trend curves of shale volume, porosity, permeability, saturation, etc.